U.S. Pat. No. 3,494,524 (Jones) discloses the principle of a rolling loop film transport mechanism. The mechanism includes a curved stator and a rotor which co-operate to define a film passage. The rotor has gaps in which loops of film are continuously formed as the rotor rotates. Each loop is developed by feeding film from an input sprocket into a gap in the rotor as the gap travels from a film inlet location to the aperture, and the loop decays progressively as the gap travels from the aperture to a driven output sprocket. Successive portions of the film (frames) are held stationary at the aperture for projection or exposure, as the case may be, on one or more registration pins.
A number of improvements in the original Jones mechanism are disclosed in U.S. Pat. No. 3,600,073 to Shaw. The disclosure of the Shaw patent is incorporated herein by reference. One such improvement involves the provision of a cam unit in advance of the aperture for engaging and decelerating the film so that the film is laid smoothly onto the registration pin or pins. In other words, the cam unit minimizes or reduces the impact forces which would otherwise be imposed on the film as the film strikes the registration pins. Those impact forces can cause damage to the film around the perforations in which the registration pins engage. This effect is particularly severe at high frame rates and with large formats.
The cam unit disclosed in the Shaw patent has movable film engaging pins that are reciprocated back and forth in the film path by a cam. As successive film loops travel past the cam unit, the film is laid onto the film decelerating pins and the pins move with the film and decelerate the film, so that the film is laid smoothly onto the registration pins at the aperture for projection or exposure as the case may be. The deceleration pins then remain in engagement with the film until the next rotor gap approaches the cam unit. As the loop in that gap passes by the cam unit, a portion of the film at the leading edge of the loop is lifted off the cam unit pins and a portion of the film at the trailing edge of the loop is subsequently laid onto those pins, ready for the deceleration cycle to be repeated. In the time that the rotor gap passes by the cam unit, the deceleration pins must be returned in a direction against the direction of film travel ready to re-engage the film.
This cycle of movement of the film decelerating pins requires abrupt acceleration and deceleration of the pins, and changes of direction that impose severe loads on the moving parts of the cam unit. This not only imposes a requirement for high precision and high quality components in the manufacture of the unit but service life maintenance requirements are high. Also, these requirements impose practical limits on the design criteria for the projector or camera itself, for example in terms of minimum rotor gap width and the frame rate at which the apparatus can be operated. In the case of a projector, it is generally desirable to operate at high frame rates so as to reduce noticeable "flicker" and to reduce the gap width so as to achieve a brighter projected image. Image brightness is affected by gap width because, in a rolling loop mechanism, the film is transported as the gap passes the aperture and each gap is therefore provided with a shutter that obscures the projected light as the gap moves across the aperture.